1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device. Note that in this specification, a semiconductor device refers to a semiconductor element itself or a device including a semiconductor element. As an example of such a semiconductor element, a transistor (a thin film transistor and the like) can be given. In addition, a semiconductor device also refers to a display device such as a liquid crystal display device.
2. Description of the Related Art
In recent years, semiconductor devices are necessary for our daily life. Semiconductor elements, such as thin film transistors, included in semiconductor devices are manufactured in such a manner that a semiconductor film is formed over a substrate and the semiconductor film is processed into a desired shape by a photolithography method or the like. Such a manufacturing method is also used for liquid crystal display devices (e.g., liquid crystal television sets), for example.
Thin film transistors of conventional liquid crystal television sets often include amorphous silicon as semiconductor films. This is because, in general, thin film transistors including amorphous silicon films can be relatively formed with ease.
As a structure of thin film transistors including amorphous silicon films, an inverted-staggered structure is widely used. A thin film transistor with an inverted-staggered structure needs fewer masks to be manufactured than thin film transistors with another structure, and accordingly, the process is simple; therefore, an inverted-staggered structure is especially advantageous in cost.
Further, a technique in which a multi-tone mask (a half-tone mask or a gray-tone mask) is used for reducing the number of masks is widely known (e.g., Patent Document 1).